1. Field of the Invention
The present invention relates generally to motor driven sewing machines. More particularly, the invention concerns an improved automatic stitching apparatus for automatically sewing the peripheral borders of work pieces of widely varying shapes.
2. Discussion of the Prior Art
Sewing machines are, of course, old in the art and have been designed to accomplish a wide variety of tasks such as utility stitching, decorative stitching, including applique, embroidery, merrow stitching and the like. Some prior art sewing machines are of relative simple construction for light sewing while others are more complex, heavy duty machines for commercial applications.
Regardless of the complexity of the particular machine, most prior art machines operate on basically the same principle and most include similar components such as a throat plate, a presser foot and feed dogs for moving the work piece relative to the needle and throat plate. Usually, stitches made by a conventional prior art sewing machine are formed by two threads which are interlocked. In the prior art vibrating shuttle type machine the upper thread is carried by the needle, while the under thread is unreeled from a bobbin. The descending needle penetrates the fabric and carries the thread along. When the needle rises again, the thread forms a loop on the underside of the fabric. The shuttle, which contains the bobbin of under thread passes through this loop and pulls the under thread along behind it. The shuttle thread is thus enclosed in the loop of the needle thread. The fabric is then moved forward usually by one or more feed dogs. During this forward movement, the needle remains stationary and the shuttle returns to its initial position. This causes the slack loop to be pulled tight so that the two threads interlock in the middle of the fabric. When the forward movement of the fabric stops, the operation is repeated. This method produces a so called lock stitch, which forms a strong but rather rigid seam.
The prior art chain-stitch type sewing machine on the other hand, produces seams having greater resilience. This type of prior art machine typically works with only one thread, which is linked at the underside of the fabric by means of a looper. A variant of this stitch is the overcast stitch which enwraps the edge of the material.
In the more modern domestic type motor driven sewing machines the so called rotary hook is frequently employed. In this type of machine the needle descends through the fabric, and the point of the hook advances to meet the needle. The return movement forms the loop, and the point of the hook enters it. The hook enlarges the loop, the front of which is held in a recess in the bobbin case, while the hook pulls away the other side of the upper thread loop over the bobbin case. The loop slips off the point of the hook while the thread take-up lever pulls the excess thread up again. During the unwinding of the thread, the side of the loop that was held in the recess is released and the loop is pulled tight.
In the typical prior art machine, rotary motion is transmitted through a top shaft to a crank drive for the thread take-up lever. In addition, the bottom shaft is driven by belts or gears from the top shaft. An eccentric cam mounted on the top shaft actuates eccentric rods and thus drives the feed mechanism or feed dogs which are positioned under a base plate and which functions to move the fabric forward. The stitch is made longer or shorter by varying the eccentric stroke and thus varying the amount of rotation that the feed motion shaft undergoes at each stroke.
Exemplary of one form of modern sewing machine is the device described in U.S. Pat. No. 4,250,824 issued to Meier et al. The method described in the Meier et al patent consists of a method for forming an overcast seam using a zigzag sewing machine with a needle bar carrying a needle with needle thread movable into first and second overstitch positions. The machine includes a looper with looper thread and a material feeder for feeding material in a feed direction. The method of the Meier et al disclosure comprises forming a first loop of needle and looper thread with the needle bar in its first overstitch position, moving the needle bar with needle thread into the second overstitch position leaving a loosened thread length between the first and second needle loop, forming a second loop of needle and looper thread with the needle in its second overstitch position forming at least one straight stitch with the needle bar in its second overstitch position in the feed direction of the material, pulling the loosened thread length between the first and second loops into the first overstitch position to form an overstitch loop into which the needle moves to from a subsequent loop, and releasing the overstitch loop.
U.S. Pat. No. 5,517,933 issued to Maraba, Jr. discloses a mechanism and method for forming a facing seam at a relatively high rate that will insure that the cover thread is properly interwoven between each stitch of two parallel rows of stitches. The facing seam is formed by two needles and cooperating loopers for forming two parallel rows of stitches that are joined by a cover thread that lies on the upper surface of the work piece. The cover thread is controlled by a conventional spreader and a cover thread assist member. The cover thread assist member is formed from a lightweight hardened wire that is fixed to a mid portion of the conventional spreader.
Somewhat more pertinent to the present invention is U.S. patent issued to Pirrello No. 3,675,602. This invention disclosed in this patent which is entitled, Automatic Emblem Sewing Machine relates to an improved sewing apparatus for automatically applying finishing stitches to the borders of successively feed work pieces such as cloth emblems. The Pirrello apparatus includes a sewing machine with an automatic needle positioner, a vacuum pickup mechanism for transferring an emblem from a stack of emblems to the sewing station, and an emblem guide mechanism including a servomotor-driven feed wheel laterally spaced from the sewing machine feed dog and controlled by an emblem edge sensor positioned rearwardly of the needle position. A stitch chain cutter actuated by the sewing machine trails the edge sensor and a stitch chain sensor trails the knife. A network controls the sequence of operations to raise the feed wheel and machine foot piece, transfer an emblem from the stack to the sewing station, to lower the wheel and foot piece, to start the sewing machine and to advance the emblem.
As will be better understood from the description which follows, the method of the present invention basically concerns the accomplishment of relatively simple but elegant modification, to a conventional type of commercially available sewing machine to enable the machine to automatically move the work piece, such as a cloth or plastic emblem, relative to the needle along a strategically positioned guide or fence in a manner to rapidly and precisely form virtually perfect finishing stitches along the border of the work piece. Although the finishing operation can be crudely accomplished using the unmodified machine, the operation is extremely time consuming and requires a high degree of operator skill which contributes greatly to the cost of the finished product.